Table 4. Nine models of gene duplication; reproduced from [5].

Model	Description	Mutation	Fitness
Ia	Extra copies of a gene are redundant andcan be relieved from purifying selection	pseudogenization and very rare new functionalization	maintained at if each gene has at least one functioning copy
Ib	Each gene has subfunctions; functionally complementary copies produce one function	mutation removes a subfunction or whole function	same as Ia, with complementary copies treated as a functioning copy
Ic	functionally complementary copies canspecialize and be more advantageous	same as Ib	specialized copy has increased fitness value
IIa	Extra copies are always beneficial	same as Ia	increase in dosage results in increase in fitness
IIb	Extra copies can shield genesagainst deleterious mutations	same as Ia; simulated with a highermutation rate	same as Ia
IIc	Gene duplication develops amodified function	mutation can introduce new functionsto the extra copies	new functions increase fitness
IIIa	Original gene carries multiple subfunctions whichcan adapt to full-fledged functions in extra copies	mutation can adapt the subfunction to full function in extra copies	extra new full function increases fitness
IIIb	Different allele types pre-exist in population;duplication and recombination togethercan create advantageous heterozygote	pseudogenization	heterozygote genes have higher fitness
IIIc	Similar to IIIc, with multi-allelic diversitybeing advantageous	pseudogenization	genes that accumulate several different alleles have higher fitness